Semiconductor and other thin film technologies often employ heated or cooled pedestals to control the temperature of workpieces prior to, during, or after processing of the workpieces. For example, a heated pedestal may be used in a processing chamber to heat and maintain a predetermined temperature of a workpiece during deposition of a layer onto the workpiece, removing materials from the workpiece surfaces, or performing other processing operations. Heated or cooled pedestals may also be provided in load locks for heating or cooling workpieces as they are being transferred in or out of the processing environment. Such pedestals may be made of aluminum or ceramic materials and formed into a single monolithic piece. A workpiece is supported above a surface of this pedestal to achieve heat transfer (i.e., heating or cooling depending on relative temperatures of the workpiece and pedestal). A gap between the pedestal surface and workpiece provides some control to this heat transfer such that a higher heat transfer rate corresponds to a smaller gap, while a lower heat transfer rate corresponds to a larger gap (i.e., the inverse proportional relationship).
Often, workpieces, particularly large but thin wafers (e.g., 450 millimeter wafers) are deformed when introduced into a processing system and need to be heated or cooled uniformly. Some common examples of such deformations include bowing, when a workpiece has a concave shape with its center portion extending downward with respect to the plane defined by its edges; and doming, when a workpiece has a convex shape with its center portion extending upward with respect to the plane defined by its edges. Deformations may also have various non-symmetrical shapes. Deformation may occur due to coefficient of thermal expansion differences among various materials forming workpieces, compressive or tensile films deposited on their surfaces, and other factors. Often workpieces in the same batch have different kinds and levels of deformation. These deformations are hard to anticipate and often random in nature. Furthermore, some deformation may occur during heat transfer, while the workpiece is already in the system. These “in process” deformations may be due to changes in workpiece temperature, deposition of additional materials, and other reasons. As such, it is difficult, and generally may not be possible to have preset heat transfer surfaces that always conform to deformed workpieces. Generally, pedestals with planar surfaces have been used because of this relatively unpredictable nature of deformations. While pedestals with predetermined curved surfaces have been proposed, their application is limited to only very specific types of deformation.
When a deformed workpiece is positioned over a planar surface of the pedestal, the gap between that surface and the workpiece will vary throughout the surface. This variation may cause non-uniform heat transfer throughout the surface, which may result in a non-uniform temperature profile of the workpiece. The temperature variation may interfere with processing and result, for example, in uneven deposition or material removal rates throughout the surface. Further, this temperature variation may cause further deformation and, in certain cases, permanent damage of the workpiece. For example, excessive deformation may cause slip dislocations in silicon structures, when portions of silicon lattice are displaced with respect to each other. This defect may degrade the electrical performance of the device. In some cases, workpieces may even break inside the apparatus, which causes prolonged shutdowns and expensive clean-ups.